Computing Progressive Failure in Materials and Structures by Integration of Digital Image Correlation with Acoustic Emission Monitoring Data

ABSTRACT

An inventive approach is disclosed to integrate Digital Image Correlation (DIC) with the Acoustic Emission method that may be used for structural health monitoring and assessment of critical structural components in civil, mechanical, and aerospace industries. The inventive approach relies on passively recording acoustic emission across the specimen being tested and activating the DIC cameras automatically to measure deformation on the specimen&#39;s surface. The resulting acousto-optic system can be used to determine damage initiation, progressive damage development, identify critical regions and make lifetime predictions of the tested specimen.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from PCT applicationPCT/US2013/37252, filed on Apr. 18, 2013, which claims priority fromU.S. Provisional Patent Application Ser. No. 61/635,282, which was filedon Apr. 18, 2012, and which are incorporated herein in their entireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a nondestructive testing and evaluation(NDT & E) system.

Description of the Related Art

Structural Health Monitoring (SHM) is vital in ensuring the structuralintegrity of critical components utilized in the aerospace, civil andmechanical industries. The development of SHM has a direct impact onpublic safety, primarily because it is beneficial in identifying earlysigns of critical failure and is related to reduced downtime and lifeextension of aging components and structures.

NDT & E systems are often a crucial part of SHM applications. Currently,no single monitoring technique has been capable of performing completestructural evaluation due to several inherent challenges includingunpredictable environmental and loading conditions, limitations of thetechniques themselves, lack of an adequately dense sensing network, etc.

Structural integrity monitoring systems based on the Digital ImageCorrelation (DIC) or the Acoustic Emission (AE) methods currently exist,however they are typically implemented independently and they areoperated manually when used to assess material and structural integrity.

Consequently, it would be beneficial to provide an integrated approachin which multiple NDT & E techniques are used to develop an effectiveSHM system.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention provides a NDT & E method of determiningthe structural integrity of a specimen or structure as external load(e.g. mechanical, thermal, environmental, etc.) is applied. The methodcomprises the steps of: attaching at least one acoustic sensor to thespecimen; applying a contrasting pattern on the surface of the specimen;calibrating a pair of stereoscopic cameras at the contrasting pattern;passively recording acoustic stress waves propagating in the specimenusing an AE system electronically coupled to the at least one acousticsensor; automatically triggering operation of the cameras by the AEsystem; measuring deformation in the specimen based on load-inducedmovement of the contrasting pattern in a DIC system, the DIC systembeing electronically coupled to the cameras; and correlating acousticstress waves and strain data to determine the structural health of thespecimen.

The present invention also provides a digital trigger signal (AE output)that is formed based on real time recorded and naturally occurringacoustic emission information, which is parametrized and used to extractfeatures, based on which the signal is formed and subsequently passedinto a DIC system. This trigger signal automatically activates thecameras of a DIC system for adaptive image acquisition depending onlyupon activity recorded nondestructively by the AE method due to changesin material and or structural integrity.

Additionally, the present invention provides time-synchronization(fusion) of data obtained by the acoustic and optical nondestructivemethods for cross-validation and interpretation of information relatedto material and/or structural integrity.

Further, the present invention provides the control of a mechanicaltesting machine, in cases when such a machine is used in conjunctionwith the combined AE and DIC systems, through a digital signal (DICoutput) defined in the DIC system based on full field deformationmeasurements that are further enabled as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this invention, illustrate the presently preferred embodimentsof the invention, and, together with the general description given aboveand the detailed description given below, serve to explain the featuresof the invention. In the drawings:

FIG. 1 is a schematic drawing of the Acousto-Optic Sensing System(“AOSS”) integrated with a mechanical test machine according to anexemplary embodiment of the present invention;

FIG. 2A is an exemplary application of the AOSS for mechanical testingof a laboratory “dog-bone” coupon subjected to uniaxial loading.

FIG. 2B is an exemplary application of the AOSS for mechanical testingof a laboratory “compact tension” coupon subjected to uniaxial loadingthat induces cracking;

FIG. 2C is an exemplary application of the AOSS for structural testingof a partially reinforced concrete masonry wall subjected to cycliclateral loading simulating earthquake excitations;

FIG. 3A is an exemplary demonstration of a DIC camera setup in the AOSSfor structural testing of a partially reinforced masonry wall subjectedto cyclic lateral loading simulating earthquake excitations;

FIG. 3B is an exemplary demonstration of the AE sensor setup in the AOSSfor structural testing of the partially reinforced masonry wall shown inFIG. 3A;

FIG. 4A is a graph overlaying stress values obtained by the AOSS withthe average full field vertical strain (true strain) calculated by theDIC system and the AE amplitude distribution extracted by real-timerecorded AE waveforms of voltage versus time, with the true stressrecorded by the AOSS using real time information from the mechanicaltesting machine;

FIG. 4B is a graph overlaying load values obtained by a mechanical testmachine with the crack length measured by the DIC system and thecumulative absolute energy computed by the recorded AE as loading isapplied to a compact tension coupon similar to FIG. 3B;

FIGS. 5A-C is an exemplary demonstration of triggering the DIC imageacquisition by using real time recorded AE information in the partiallyreinforced concrete masonry wall application shown in FIG. 2C and FIG.3, wherein

FIG. 5A shows the specific AE feature (cumulative energy) as a functionof time and correlated with load recorded by a mechanical test machineand also read by the AOSS trends data;

FIG. 5B shows representative images with full field surface straindistribution showing crack initiation;

FIG. 5C shows similar surface strain distribution on the masonry wallthat corresponds to the second time instance marked as “Critical CrackGrowth” in FIG. 5A;

FIG. 6A is a graph demonstrating time-synchronization of informationcollected by the AOSS by using data obtained by the DIC (strain), AE(Partial Power 3) and an added infrared thermography system (surfacetemperature) in an exemplary application in the case of the compacttension coupon test shown in FIG. 2B;

FIG. 6B is an enlargement of a portion of the graph of FIG. 6A marked bythe broken line that shows the time synchronization of the threedifferent datasets achieved by the AOSS;

FIG. 7 is an exemplary application of integrating information (datafusion) obtained by the AOSS for damage diagnosis in the case of thecompact tension coupon test shown in FIG. 2B, with the Mahalanobisdistance being defined as a parameter using information from both DICand AE in the AOSS;

FIG. 8A is a graph highlighting the correlation between DIC (residualstiffness) and AE (cumulative energy) data with lifetime fractions of alaboratory coupon similar to the laboratory coupon shown in FIG. 2A; and

FIG. 8B is another graph highlighting the correlation between DIC(energy density) and AE (cumulative energy) data with lifetime fractionsof a laboratory coupon.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, like numerals indicate like elements throughout.Certain terminology is used herein for convenience only and is not to betaken as a limitation on the present invention. The terminology includesthe words specifically mentioned, derivatives thereof and words ofsimilar import. The embodiments illustrated below are not intended to beexhaustive or to limit the invention to the precise form disclosed.These embodiments are chosen and described to best explain the principleof the invention and its application and practical use and to enableothers skilled in the art to best utilize the invention.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

As used in this application, the word “exemplary” is used herein to meanserving as an example, instance, or illustration. Any aspect or designdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other aspects or designs. Rather, use ofthe word exemplary is intended to present concepts in a concretefashion.

Additionally, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or”. That is, unless specified otherwise, or clearfrom context, “X employs A or B” is intended to mean any of the naturalinclusive permutations. That is, if X employs A; X employs B; or Xemploys both A and B, then “X employs A or B” is satisfied under any ofthe foregoing instances. In addition, the articles “a” and “an” as usedin this application and the appended claims should generally beconstrued to mean “one or more” unless specified otherwise or clear fromcontext to be directed to a singular form.

The present invention includes an optical and an acoustic method toadaptively obtain full field deformation measurements on a material's orstructure's surface based on a signal formed by extracting informationfrom volume-related measurements of naturally occurring acousticemission. This invention, referred to here as “Acousto-Optical SensingSystem (AOSS)”, describes the communication between its constituents andbasis of operation using the Digital Image Correlation (DIC) andAcoustic Emission (AE) methods. The inventive system and method featuresan approach to activate and trigger recordings in cameras related to DICbased on signals formed and exported by the AE method. This approach canbe used to time-synchronize full field deformation measurements withemission of acoustic stress waves occurring in the inspected volume of amaterial and/or structure by natural causes such as evolving damage dueto the application of external loadings, such as, for example,mechanical, thermal, environmental, and other types of loadings.

The present invention also provides a novel approach of integrating inreal testing time the DIC with the AE method both at the hardware and atthe post processing/analysis levels. The hardware integration betweenthe two techniques is based on user-defined, multi-parametric criteriain the AE system that are used to form a digital output signal. Uponreceiving this signal, the DIC cameras are automatically activated andtriggered to acquire images based on a user-built script. Consequently,the unique advantage of this hardware integration is an adaptiverecording and therefore also memory-storage effective NDE approach thatcan time-synchronize acoustic with optical information related tomaterial and/or structural failure.

An exemplary AOSS system 100 according to the present invention is shownin FIG. 1. A mechanical test stand 110 is used to apply mechanical loadto a test specimen 112, which simulates an object under load. Suchobject may be a bridge beam, a building column, an airframe, or otherstructural element that may be under load by its normal operationalenvironment. The load may be a compressive load, a tensile load, atorsional load, a bending load, environmental load, fatigue load or anyother load that may be experienced by structural elements. In thisparticular embodiment, test stand 110 provides axial tensile/compressiveloads to a test specimen 112.

Exemplary test specimen 112 is shown in FIG. 2A. Test specimen 112 is analuminum alloy designed according to (ASTM) standards, although thoseskilled in the art will recognize that test specimen 112 can be othermaterials as well. Test specimen 112 a in FIG. 2B is another example ofthe same material with a different specimen geometry based on AmericanSociety for Testing and Materials (“ASTM”) standards. Test specimen 113shown in FIG. 2C is a partially reinforced concrete masonry wall todemonstrate the applicability of the AOSS in several materials and atdifferent length scales.

An exemplary AE data acquisition system includes the four-channel DiSPsystem 114 developed by Physical Acoustics Corporation, schematicallyshown in FIG. 1. AE system 114 is equipped with four piezoelectrictransducers 115-118 and associated preamplifiers 119-122. While a singletransducer 115 may be used, additional transducers 116-118 may be usedas receivers that passively receive acoustic stress waves propagatingthrough test specimen 112, 113 during testing. Further, whilepiezoelectric transducers are used, other sensors technologies, such asfiber bragg and MEMS sensors can be used. Such sensing technologies arereferred to as “acoustic sensors” in the remainder of this text. Thesystem 114 shown in FIG. 1 has analog inputs that allow load anddisplacement/strain recordings from the mechanical test stand 110.

The exemplary DIC data acquisition system 123 is a GOM ARAMIS 3D5-megapixel camera system with analog inputs that allow load recordingfrom the mechanical test stand 110. In addition, the data acquisition ofthe DIC system 123 also supports input/output ports that can trigger thecameras to activate and record images based on an external trigger.Exemplary 5-megapixel cameras are Baumer TGX15 124-125, shown in FIG.3A.

The AE system 114 is electronically connected to the DIC system usingBNC cables and an external parametric box, shown at 101 in FIG. 1, sothat information in the form of input/output signals between the AEsystem 114 and the DIC system 123 can be exchanged in real experimentaltime. An exemplary

The direct connection between the AE system 14 and the DIC system 123enables the DIC system 123 to be automatically activated and triggeredto acquire images based on a TTL signal generated by the AE system 114.Additionally, the AE system 114 is connected using BNC cables 102 totest stand 110 and to a load cell 105 using BNC cables 103. Thus, the AEsystem 114 is also equipped to receive load/displacement or any otherparametric input in real time through a parametric box. Further, the DICsystem 123 is connected to the load cell 105 by BNC cables 104. A closedloop is formed between test stand 110, DIC system 123, and AE system 114so that information (for example, load) recorded from one system can bepassed to other systems (for example AE and DIC) and synchronized atboth the time and loading stages.

For DIC measurements, a contrasting speckle pattern must be present onthe surface of test specimens 112-113. In this case, a random specklepattern is applied on the surface of test specimens 112-113 for trackingdeformation, and pretest images of test specimens 112-113 are taken todetermine the sensitivity of system 123 for a particular field of view.The random speckle pattern is used to identify the relative displacementof test specimens 112-113 by correlating the acquired images to a knownreference image under load.

In an alternative embodiment, such as, for example, determining strainin a bridge beam (not shown), if natural surface contrasts are readilypresent on the beam, such as, for example, dirt, paint chips, or anyother random pattern, then the random speckle pattern does notnecessarily need to be applied to the beam. In such a situation, a loadis already present in the beam and the piezoelectric transducers 115-118are attached to the beam after the load has been applied to the beam.

As an exemplary method, true stress (calculated by using data recordedby the load cell of the test stand) versus true strain (calculated byusing the DIC system), while AE data (AE waveform amplitudedistribution) has been synchronized and added to the true stress-truestrain curve in FIG. 4A. As an additional exemplary method, crack lengthmonitoring (computed by the DIC system) as a function of applied (by thetest stand) load and the cumulative absolute energy (computed by the AEsystem) is shown in FIG. 4B. The AE system and the features extracted orcomputed by it were used in the examples shown to activate and recorddata by the DIC system.

Additional exemplary correlations between DIC, AE and mechanical testdata are shown in FIG. 5A in case of cyclic loading of a structuralcomponent. The cumulative energy computed by the AE system issynchronized with load information and is used to trigger the cameras ofthe DIC system that record full field images of surface deformation (inthis case strain) as shown in FIG. 5.

FIG. 5A shows the specific AE feature (cumulative energy as a functionof time and correlated with load recorded by the mechanical test machineand also read by the AOSS trends data. Note the two time instancesdenoted by the two vertical broken lines and labeled as “CrackInitiation” and “Critical Crack Growth”. Changes in the AE features atthis time instances were used to trigger the DIC system and record fullfield surface strain information as shown in FIGS. 5B and 5C.

FIG. 5B shows representative images with full field surface straindistribution showing crack initiation, noted by the box “B” on thebottom left corner of the Figure, which corresponds to the first timeinstance marked in FIG. 5A. FIG. 5C shows similar surface straindistribution on the masonry wall that corresponds to the second timeinstance marked as “Critical Crack Growth” in FIG. 5A. The lines marked“C” correspond to regions on the wall with more pronounced crackformation and grown in a staircase pattern.

In FIGS. 6A and 6B, the measured strain by the DIC system afterreceiving feedback from the AE system is plotted against a calculated AEfeature (partial power, which is an AE parameter that can be extractedfrom digital signal processing) as well as the surface temperaturechange of the specimen measured by an addition to the exemplary AOSSwhich in this case also comprises an infrared thermography camera (notshown).

In FIG. 7, a parameter based on what is known as Mahalanobis distancewas calculated by using features recorded by both the AE and DIC system,after the DIC system was triggered by the AE system; this parameter isused to detect the extent of damage (in this case length of crack in anexperiment similar to the experiment illustrated in in FIG. 2B).

In FIG. 8 an exemplary application of how the demonstrated AOSS is usedto predict the life fraction of a material or structural component ispresented by plotting parameters computed by using DIC data (residualstiffness in FIG. 8A and energy density in FIG. 8B) with the cumulativeenergy computed based on AE data. Note that DIC data were recorded aftertriggering of the DIC system using an input signal based on changes ofthe AE data.

The advantages of the inventive Acoustic-Optical Sensing system includeseamless integration of AE with DIC, in that AE and DIC are now capableof communicating with each other without manual intervention.Consequently, this novel setup provides deformation measurements throughthe DIC system based on criteria defined in the AE system. Further, thenovel system provides the capability to acquire DIC imagery only whenidentified by the operator as “critical” AE information is recorded,thereby enabling digital memory savings in the DIC system. This aspectcould be particularly useful in long-term SHM applications.

Additionally, the novel AOSS provides the opportunity to integrate fullfield mechanical parameters such as in- and out-of-plane deformationmeasurements and material properties including Poisson's ratio withtime, frequency and joint time-frequency AE features such as amplitude,peak frequency, partial powers, and other known parameters. Thiscombination of information enables a cross-validated evaluation ofmaterial and/or structural integrity which increases the reliability ofthe measurements recorded by each of the two systems independently.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1-20. (canceled)
 21. A nondestructive method of determining thestructural integrity of a specimen as a load is applied to the specimen,the method comprising the steps of: attaching at least one acousticemission sensor to the specimen; applying a contrasting pattern on thesurface of the specimen; aiming a pair of stereoscopic cameras at thecontrasting pattern; passively recording acoustic stress wavespropagating in the specimen in an Acoustic Emission (AE) systemelectronically coupled to the at least one acoustic emission sensor;time synchronizing cumulative energy calculated from the AE data withinformation about the load; triggering operation of the cameras by theAE system; adaptively recording data generated by operation of thecameras; measuring deformation in the specimen based on load-inducedmovement of the contrasting pattern in a Digital Image Correlation (DIC)system, the DIC system being electronically coupled to the cameras; andcorrelating stress waves travelling through the specimen measured by theat least one acoustic emission sensor with strain data measured on asurface of the specimen to compute a Mahalanobis distance of thespecimen.
 22. The method according to claim 21, further comprising,after calibrating the cameras at the contrasting pattern, applying aload to the specimen.
 23. The method according to claim 22, wherein theload applying step comprises attaching the specimen to a load applyingtest stand.
 24. The method according to claim 23, wherein the at leastone acoustic emission sensor and the cameras are electrically connectedto the test stand.
 25. The method according to claim 24, wherein adigital output is sent from the AE system to the DIC system.
 26. Themethod according to claim 25, further comprising triggering the DICcameras based on an input signal received from the AE system.
 27. Themethod according to claim 26, wherein the correlating step comprisesmeasuring deformation in the specimen synchronized with the applied loadand acoustic activity.
 28. The method according claim 27, wherein thecorrelating step comprises computing a life fraction of the specimen byusing synchronized DIC and AE data.
 29. The method according to claim21, wherein the at least one acoustic sensor comprises a plurality ofacoustic sensors.
 30. A method of determining the structural integrityof a specimen as a load is applied to the specimen, the methodcomprising the steps of: prior to applying the load: attaching at leastone acoustic emission sensor to the specimen; determining a contrastingpattern on the surface of the specimen; and aiming two cameras at thecontrasting pattern; determining strain in the specimen based onload-induced movement of the contrasting pattern; passively recordingacoustic stress waves propagating in the specimen; and correlatingstress waves travelling through the specimen with strain data measuredon a surface of the specimen to determine structural health of thespecimen and to compute a Mahalanobis distance for the specimen.